Continuity mistake: Towards the end, when the scientist is trying to give everybody diamonds, a shot cuts to just Marge and Lisa, but out of nowhere Maggie appears. (00:19:30)
Continuity mistake: When they are riding the boat type raft, the stripes change from red on the left and black on the right, to the other way around and then back again.
Continuity mistake: When Bart is eating the grubs, his tongue appears to be green, in all other shots it's back to red.
Continuity mistake: When the Simpsons are on the raft going down the river in Africa they come to a fork and have a choice of going to the left, towards a sunny and nicer area or to the right where it is rainy and gloomy. As Homer tries to steer the raft to the left as he pulls his left arm out of the water and it is covered in leeches but in the next shot of him when we see the arm again the leeches are gone. No way to remove leeches that fast.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.